1. Field of the Invention
The present invention generally relates to novel working fluids uniquely designed for higher cycle efficiencies, thereby leading to higher overall system efficiencies and therefore lower fuel consumption and lower emissions levels. In particular, these working fluids are useful in Rankine cycle systems for efficiently converting waste heat generated from industrial processes, such as electric power generation from fuel cells, into mechanical energy.
2. Discussion of the Background Art
Rankine cycle systems are known to be a simple and reliable means to convert heat energy into mechanical shaft power. Organic working fluids are useful in place of water/steam when low-grade thermal energy is encountered. Water/steam systems operating with low-grade thermal energy (typically 400° F. and lower) will have associated high volumes and low pressures. To keep system size small and efficiency high, organic working fluids with boiling points near room temperature are employed. Such fluids would have higher gas densities lending to higher capacity and favorable transport and heat transfer properties lending to higher efficiency as compared to water at low operating temperatures. In industrial settings there are more opportunities to use flammable working fluids such as toluene and pentane, particularly when the industrial setting has large quantities of flammables already on site in processes or storage. For instances where the risk associated with use of a flammable working fluid is not acceptable, such as power generation in populous areas or near buildings, other fluids such as CFC-113 and CFC-11 were used. Although these materials were non-flammable, they were a risk to the environment because of their ozone-depletion potential. Ideally, the organic working fluid should be environmentally acceptable, non-flammable, of a low order of toxicity, and operate at positive pressures.
Organic Rankine cycle systems are often used to recover waste heat from industrial processes. In combined heat and power (cogeneration) applications, waste heat from combustion of fuel used to drive the prime mover of a generator set is recovered and used to make hot water for building heat, for example, or for supplying heat to operate an absorption chiller to provide cooling. In some cases, the demand for hot water is small or does not exist. The most difficult case is when the thermal requirement is variable and load matching becomes difficult, confounding efficient operation of the combined heat and power system. In such an instance, it is more useful to convert the waste heat to shaft power by using an organic Rankine cycle system. The shaft power can be used to operate pumps, for example, or it may be used to generate electricity. By using this approach, the overall system efficiency is higher and fuel utilization is greater. Air emissions from fuel combustion can be decreased since more electric power can be generated for the same amount of fuel input.
U.S. Pat. No. 6,365,289, which is incorporated in its entirety, articulates that an organic Rankine cycle can be used to produce shaft work to operate fuel cell system components, such as an air compressor, in order to produce compressed air used in the fuel cell stack. It states that the working fluid can be heated by an external boiler or by one of the components of the fuel cell system, such as the combustor and/or the fuel cell stack. This treatment does not address utilization of waste heat for the purpose of increasing overall system efficiency or the management of fuel cell system temperatures which would provide improved process stability and system reliability/integrity, such as that disclosed hereinafter by the present invention.
One of the benefits of the present invention is that the present inventors have unexpectedly determined that the use of an organic Rankine cycle scaled to utilize the available fuel cell waste heat can generate additional electric power. Because the electrical power output of the fuel cell and appended organic Rankine cycle system is greater than that of the fuel cell alone and the fuel energy input is constant, the result of the present invention is an increase in overall thermal efficiency. Organic Rankine cycle offers an advantage over combined heat and power (cogeneration) processes in that no load matching is required. In many cases, load matching can be difficult to accomplish. In cases where there is no thermal requirement, a combined heat and power process could not be utilized; however, an appended organic Rankine cycle system remains an advantageous means to improve fuel cell system overall efficiency.
Currently, fuel cell development and commercialization is underway. Various types of fuel cells are being used for power generation at scales ranging from utility power generation down to power supplies for portable computers. Some fuel cell designs, such as proton exchange membrane types, do not generate significantly high temperatures, around 100° C. or so, and would not have adequate levels of thermal energy for use of organic Rankine cycle. Other designs, such as phosphoric acid fuel cells, operate at moderate temperatures and would lend well to the use of organic Rankine cycle systems for thermal energy conversion. Rankine cycle systems running with water or silicones as the working fluid are efficient when the waste heat is available at high temperature. High temperature fuel cells such as molten carbonate or solid oxide fuel cells would be an example. Nonetheless, organic working fluids with adequate thermal stability and low-to-moderate vapor pressure can be used practically to utilize waste heat from high-temperature sources provided organic Rankine cycle systems are properly designed. For example, the heat recovery heat exchanger would be designed to provide high flow rates and low heat flux in the first portion of the heat exchanger thus protecting the organic Rankine cycle working fluid from thermal decomposition until the source-side temperature was brought down to that where work extraction could take place for the given working fluid.
Fuel cells are meant to be a reliable, environmentally friendly source of electric power. Although flammable or combustible working fluids can be used in organic Rankine cycle systems, when considering fuel cells that integrate Rankine cycle systems to improve overall efficiency, selection of a non-flammable, low toxicity, environmentally friendly working fluid is an important factor. A non-flammable, low toxicity fluid contributes to reliability and safety. A non-flammable working fluid will not likely jeopardize the system or fire safety of the surroundings if internal or external forces or events such as ignition sources interact with the working fluid. Fluids with a low order of toxicity and favorable environmental properties such as low global warming potential bring less impact to the environment and its inhabitants if there are exposures or releases.
The working fluids of the present inventions uniquely provide higher cycle efficiencies in Rankine cycle systems that in turn result in higher overall system efficiencies and therefore lower fuel consumption and lower emissions levels.